The present invention relates to a light guide plate for use as a backlight plate in a liquid crystal display device or an emission guide plate.
Recently, the present applicant has proposed a planar surface illuminator to be used for backlight in liquid crystal displays as shown in FIG. 6 (Japanese Patent Laid-Open Publication HEI 09-325218). This planar surface illuminator 31, which is provided below a liquid crystal display panel 40, comprises a light guide plate 32 made of transparent plastic resin and having a diffraction grating 33 provided on a rear surface 32b, a fluorescent tube 34 having a cold cathode or semi-hot electrode as a light source placed along a thicker end side edge 32c of the light guide plate 32, a reflector 35 which surrounds and covers the light guide plate, except its front surface 32a, as well as the fluorescent tube 34 to reflect light, a diffusion plate 36 placed parallel to the light guide plate on its front surface 32a side, and a prism sheet 37 for light collection placed parallel to the diffusion plate 36 on its front surface side.
The diffraction grating 33 is formed by molding in the form of minute ruled grooves on the rear surface 32b which is inclined at an angle of 0.5 to 5.degree. so as to be able to receive on its entire surface light that comes incident generally horizontally from the fluorescent tube 34, where the distance d between adjacent grooves in the diffraction grating 33 is set so that diffracted light rays of low-order according to a later-described relational equation (1) of diffraction go out from the front surface 32a of the light guide plate generally vertically and in a direction coincident with the direction of total reflection. Also, a ratio of grating part width/non-grating part width in a unit width of the diffraction grating 33 (heavy-line length/thin-line length of each division) schematically shown as eleven divisions in the figure is set so as to increase gradually with increasing distance from the end edge 32c in order that the quantity of diffracted light increases with decreasing quantity of light that comes up from the fluorescent tube 34. In addition, the divisions are actually provided in numbers far greater than eleven, for example, around 1000.
In this planar surface illuminator 31 of the prior art, white light emitted from the fluorescent tube 34 enters the light guide plate 32 generally horizontally at the end edge 32c, impinges on the whole rear surface 32b inclined at an angle of 0.5 to 5.degree., and is diffracted by a synergistical cooperating effect of adjacent smooth surfaces between each numerous ruled grooves of the diffraction grating 33 which is provided over the whole rear surface 32b and whose groove interval, namely grating constant d is on the order of submicrons to several tens of microns (0.1 to 10 .mu.m), so that high intensity diffracted light of low-order (e.g., 1st- to 3rd-order) go out generally vertically from the front surface 32a of the light guide plate 32 as shown by an arrow in the figure. Thus, far higher-intensity outgoing light can be obtained, as compared with the conventional multiplicity of trigonal-pyramid prism surfaces in which one side edge is long as much as 0.16 mm, and at which each ray of light is totally reflected as a sum of light quanta geometrically-optically and individually without cooperation with neighborhoods. In addition to this, because grating part width/non-grating part width in unit width of the diffraction grating 33, i.e. the diffraction efficiency of the grating (ratio of diffracted light intensity to incident light intensity), increases with increasing distance from the end edge 32c on the fluorescent tube 34 side, the quantity of diffracted light increases in proportion to decreases in the quantity of light due to increasing distance from the light source. In this way, the front surface 32a of the light guide plate 32 is illuminated with high brightness and great uniformity.
It is noted that since the white light emitted from the fluorescent tube 34 has a spectral distribution having peaks at blue (B), green (G) and red (R), the diffracted light is separated as shown by arrows R, G and B in FIG. 6 according to a later-described relational equation (1) of diffraction, but changed into the original white light by passing through the diffusion plate 36 placed in the front and then collected by the prism sheet 37 placed in the front, thus going out. As a result, the liquid crystal display panel 40 is illuminated with separation-free white light from below with high brightness and yet uniformity.
Also, the light guide plate 32 except its front surface as well as the fluorescent tube 34 are covered with the reflector 35. Therefore, almost all the light of the fluorescent tube 34 is caused to be incident on the light guide plate 32, so that the liquid crystal display panel 40 is illuminated with even higher brightness.
As an experimental example of high-brightness, uniform illumination of the light guide plate 32, a light guide plate having a diffraction grating with d=3 .mu.m was fabricated by using a mold which is formed with a ruled pattern by micromachining, and surface brightness at a position 100 mm distant from an end edge on the light source side was compared with similar surface brightness of a conventional light guide plate having an about 300 .mu.m print pattern. This comparison has proved that the former surface brightness is twice brighter than the latter. Accordingly, this light guide plate 32 is capable. of offering high-brightness backlight even with a fluorescent tube 34 involving less power consumption. Therefore, applying the light guide plate 32 to battery-driven liquid crystal displays allows the life of the batteries to be prolonged double, and applying the light guide plate 32 to battery-driven liquid crystal televisions enables image watching in the light open air.
For the prior-art light guide plate 32 proposed by the present applicant, it is a presumption for high-brightness, uniform planar illumination that the fluorescent tube 34 is extending along the end edge 32c of the light guide plate 32, so to speak, as a line light source.
However, it has been becoming more often the case that the fluorescent tube 34 cannot be used due to the demand for more compactness of the light guide plate 32 in keeping with downsizing of the liquid crystal display panel 40 or for savings in power consumption so that several small, power-saving point light sources such as light-emitting diodes must inevitably be used to make up the backlight.
Unfortunately, for example, when three light-emitting diodes, arranged at the thicker end side edge 32c of the conventional light guide plate 32 as shown in FIG. 2, are lit, the result would be only that three bright lines L extending in straight line longitudinally from the light-emitting diodes appear on the front surface 32a of the light guide plate 32. This brightness could not be spread laterally, so that intermediate portions between adjacent bright lines L a nd L would be dark, causing a problem that liquid crystal display panels sized over about 2.times.4 inches could not be illuminated with high brightness and uniformity.